That depends on the game. Older games such as FEAR should run fine at that resolution with the 320mb version, but newer games such as STALKER or mabye even Oblivion will probably require a 640mb 8800GTS.
If in doubt always go for more texture memory at that resolution. Any you don't use can go towards the AA. I have oblivion running@1080p on a 1950pro, and I'm certain that the only reason it isnt horrible is because of the 512Mb ram over 256Mb. At these resolutions the GPU brand is only part of the story for getting the best out of your system.
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